My head is getting tangled looking at frame sizes.
Look at this guide:
For a 6 foot bloke the size suggested is 19-20in or 55-57cm. But those measurements aren't equal - 20 inches is less than 51cm.
This one is even weirder:
It suggests 17in or 55-57cm, which are absolutely nothing like the same length. 17 inches is only 43cm.
Am I missing something obvious here?